1. Field of the Invention
The present invention relates to the passivation of Group III-V surface layers and in particular, GaAs.
2. Description of the Prior Art
Gallium arsenide and other Group III-V type semiconductor materials are highly valued for fabricating high performance semiconductor components. These materials, however, are susceptible to environmental degradation. The poor electric quality of the degraded III-V surfaces is caused by the high density of the surface states thereon created by oxidation reactions. Deep level traps are produced as a result of oxidation which pin the Fermi level and increase non-radiative recombination.
These arsenic atoms are the main cause for the deep level traps which pin the Fermi level and increase the non-radiative recombination [Aspnes, Surf. Sci., Vol. 132, p 406 (1983); Henry et al , J. Appl. Phys. Vol. 49, p.3530 (1978); Chang et al, Appl. Phys. Lett., Vol. 33, p. 341 (1978)]. This inherent problem has limited the performance of existing GaAs based electronic and electro-optical devices and has still prevented the successful development of GaAs based electronic and electro-optical devices and has still prevented the successful development of GaAs-based MIS technology.
The principal culprit in degradation of III-V surfaces is the presence of oxygen that is difficult to exclude. In the case of GaAs, the presence of oxygen causes oxidation of GaAs to arsenic oxide and gallium oxide. The gallium atoms in the vicinity of the arsenic oxide gradually extract oxygen from arsenic oxide to form gallium oxide, leading to the segregation of the arsenic atoms.
Recently, there has been a renewed interest in improving the poor electronic quality of GaAs surface [Offsey et al, Appl. Phys. Lett., Vol. 48, p. 475 (1986); Sandroff et al, Appl. Phys. Lett., Vol. 51, p. 33 (1987); Skromme et al, Appl. Phys. Lett., Vol. 51, p. 2022 (1987) and Yablonovitch et al, Appl. Phys. Lett., Vol. 51, p 439 (1987)] which is caused by the high density of surface states on GaAs formed by segregated arsenic atoms [Spicer et al, J. Vac. Sci., Techol., Vol. 16, p. 1422 (1979)] via oxidation reactions [Lee et al, Electrochem., Vol. 135, p. 496 1988)].
Known surface passivation methods for III-V compound semiconductors of which GaAs is the most important and the most difficult to passivate are of three types.
The first method utilizes deposited films such as SiO.sub.2, Si.sub.3 N.sub.4, Al.sub.2 O.sub.3 and P.sub.2 O.sub.5 which are known from their use as passivation films for the surfaces of silicon semiconductors. These approaches suffer from the drawback that the deposition temperature is relatively high. SiO.sub.2 films tend to take in Ga from the surface of a substrate made of GaAs or GaP, and will damage the stoichiometry of the surface of the substrate.
The second method is to form a native oxide film corresponding to a thermal oxidation film of silicon, in place of the deposited film suggested above by, for example, an anodic oxidation method. Anodic oxidation methods have the disadvantage that they are thermally unstable. The quality of the film will change substantially at a temperature below the temperature range adopted for thermal diffusion of impurities and post-ion implantation annealing.
The third approach is to perform chemical oxidation by the use of, for example, hot hydrogen peroxide solution. This method is disadvantaged by limitation in the thickness of the oxide film which is formed.
The success of any passivating technique hinges on choosing a species that makes a strong bond with the surface and at the same time has a higher heat of oxide formation than that of gallium oxide since the thermodynamic equilibrium of the composition greatly favors the formation of gallium oxide over arsenic oxide because of the higher heat of formation of gallium oxide. The first requirement can be satisfied by choosing a chemical species that absorbs strongly on the GaAs surface as an impenetrable passivating barrier. Thus, a species that chemisorbs strongly must be chosen. A chemisorbing species such as phosphorus compounds occupies the active surface sites, thereby preventing adsorption of other species.
It is an object of the invention to provide a novel and efficient method of passivating Group III-V surfaces that does not suffer from the drawback associated with prior art of passivation methods.